1. Field of the Invention
The present disclosure relates to a thin film transistor substrate having a metal oxide semiconductor for flat panel display and a method for manufacturing the same. Especially, the present disclosure relates to a thin film transistor having a metal oxide semiconductor in which the stability of the channel layer is enhanced by the thermal treatment and a method for manufacturing the same.
2. Discussion of the Related Art
Nowadays, as the information society is developed, the requirements of displays for representing information are increasing. Accordingly, the various flat panel displays are developed for overcoming many drawbacks of the cathode ray tube such as heavy weight and bulk volume. The flat panel display devices include the liquid crystal display device (or LCD), the field emission display (or FED), the plasma display panel (or PDP), the electroluminescence device (or ED) and the electrophoretic display device (or EDD).
The display panel of a flat panel display may include a thin film transistor substrate having a thin film transistor allocated in each pixel area arrayed in a matrix manner. For example, the liquid crystal display device represents video data by controlling the light transmitivity of the liquid crystal layer using the electric fields. According to the direction of the electric field, the LCD can be classified in the two major types; one is vertical electric field type and the other is the horizontal electric field type.
For the vertical electric field type LCD, a common electrode formed on an upper substrate and a pixel electrode formed on a lower substrate are facing with each other for forming an electric field of which direction is perpendicular to the substrate face. A twisted nematic (TN) liquid crystal layer disposed between the upper substrate and the lower substrate is driven by the vertical electric field. The vertical electric field type LCD has merit of higher aperture ratio, while it has demerit of narrower view angle about 90 degree.
For the horizontal electric field type LCD, a common electrode and a pixel electrode are formed on the same substrate in parallel. A liquid crystal layer disposed between an upper substrate and a lower substrate is driven in In-Plane-Switching (IPS) mode by an electric field parallel to the substrate face. The horizontal electric field type LCD has a merit of wider view angle over 160 degrees and faster response speed than the vertical electric field type LCD. However, the horizontal electric field type LCD may have demerits such as low aperture ratio and transmitivity ratio of the back light. In the IPS mode LCD, for example, in order to form the in-plane electric field, the gap between the common electrode and the pixel electrode may be larger than the gap between the upper substrate and the lower substrate, and in order to get enough strength of the electric field, the common electrode and the pixel electrode may have a strip pattern having certain width. Between the pixel electrode and the common electrode of the IPS mode LCD, the electric field horizontal with the substrate is formed. However, just on the pixel electrode and the common electrode, there is no electric field. That is, the molecules disposed just over the pixel common electrodes are not driven but maintain the initial conditions (the initial alignment direction). As the liquid crystal in the initial condition cannot control the light transmitivity properly, the aperture ratio and the luminescence may be degraded.
For resolving these demerits of the IPS mode LCD, the fringe field switching (or FFS) type LCD driven by the fringe electric field has been proposed. The FFS type LCD comprises the common electrode and the pixel electrode with the insulating layer there-between, and the gap between the pixel electrode and the common electrode is set narrower than the gap between the upper substrate and the lower substrate. So that, a fringe electric field having a parabola shape is formed in the space between and on the common electrode and the pixel electrode. Therefore, liquid crystal molecules disposed between the upper substrate and the lower substrate can be driven by this fringe field. As a result, it is possible to enhance the aperture ratio and the front luminescence.
FIG. 1 is a plane view illustrating a thin film transistor substrate having an oxide semiconductor layer included in a fringe field type liquid crystal display according to the related art. FIG. 2 is a cross-sectional view illustrating the structure of the thin film transistor substrate of FIG. 1 by cutting along the line I-I′ according to the related art.
The thin film transistor substrate shown in FIGS. 1 and 2 comprises a gate line GL and a data line DL crossing each other with a gate insulating layer GI therebetween on a lower substrate SUB, and a thin film transistor T formed at the crossing portion. By the crossing structure of the gate line GL and the data line DL, a pixel area is defined. In the pixel area, a pixel electrode PXL and a common electrode COM facing each other with a passivation layer PAS therebetween are disposed for forming the fringe field. For example, the pixel electrode PXL has a rectangular shape corresponding to the shape of the pixel area, and the common electrode COM has a plurality of strips disposed in parallel each other.
The common electrode COM is connected to a common line CL disposed in parallel with the gate line GL. A reference voltage (or common voltage) is supplied to the common electrode COM through the common line CL.
The thin film transistor T charges and maintains the pixel signal voltage to the pixel electrode PXL by responding to the gate signal of the gate line GL. To do so, the thin film transistor T comprises a gate electrode G branched from the gate line GL, a source electrode S branched from the data line DL, a drain electrode D facing the source electrode S and connecting to the pixel electrode PXL, and an active layer A overlapping with the gate electrode G on the gate insulating layer GI for forming a channel between the source electrode S and the drain electrode D. Between the active layer A and the source electrode S, and between the active layer A and the drain electrode D, there may be further the ohmic contact layers.
The active layer A is made of oxide semiconductor material, as it has high electron mobility characteristics so it is good for the large area thin film transistor substrate which requires larger charging capacitance. However, the oxide semiconductor materials are not developed for the electrical elements to have good characteristics with the stabilized conditions. Therefore, it is preferable to have an etch-stopper ES on the active layer A for protecting the oxide semiconductor material. For example, in the step for patterning the source electrode S and the drain electrode D by photo-lithography method, the active layer A can be protected from the etching material by forming the etch stopper ES between the source electrode S and the drain electrode.
At one end portion of the gate line GL, a gate pad GP is formed for receiving the gate signal from the external video device. The gate pad GP is connected to the gate pad terminal GPT through the gate pad contact hole GPH penetrating the gate insulating layer GI and the passivation layer PAS. Further, at one end portion of the data line DL, a data pad DP is formed for receiving the data signal from the external video device. The data pad DP is connected to the data pad terminal DPT through the data pad contact hole DPH penetrating the passivation layer PAS.
The pixel electrode PXL disposed on the gate insulating layer GI is connected to the drain electrode D. Further, the common electrode COM is formed to overlap with the pixel electrode PXL there-between the passivation layer PAS covering the pixel electrode PXL. The electric field can be formed between the pixel electrode PXL and the common electrode COM, and then the liquid crystal molecules horizontally disposed between the thin film transistor substrate and the color filter substrate can be rotated by the dielectric anisotropy. According to the rotating conditions of the liquid crystal molecules, the light transmitivity of the light through the pixel area can be controlled and the various gray scales can be represented.
Even though the thin film transistor substrate having the metal oxide semiconductor material may have many merits, the stability of the oxide semiconductor material is not ensured. Therefore, there are many obstacles for developing the electric elements using the oxide semiconductor material.